In general, carbohydrates can be produced from lignocellulosic natural materials by hydrolysis of poly- and oligosaccharides. Apart from a total hydrolysis process only leaving a lignin residue, a combination of hydrolysis and cellulose pulp cooking has been developed, called prehydrolysis pulping. The main emphasis has been on the pulp, reflecting the business incentives. In prior art processes, hemicelluloses are hydrolysed into hydrolysate, and lignin is dissolved by a cooking method for liberating cellulose fibers. The produced pulp has a high content of alpha cellulose and can be used e.g. as dissolving pulp.
From a historical perspective, there are two processes for the production of special pulps having a high content of alpha cellulose: the far-extended acidic bisulfite cooking and the prehydrolysis-sulfate (kraft) cooking. The former was developed at the beginning of the 20th century and the latter in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, p. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibers in connection with the de-lignification so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure. Kraft prehydrolysis pulping processes are disclosed in e.g. Canadian patent application 1,173,602 (Arhippainen et al.) and in U.S. Pat. Nos. 5,589,033 (Tikka and Kovasin), 5,676,795 (Wizani et al.) and 4,436,586 (Elmore).
In the traditional sulfite process, the removal of hemicellulose takes place during the cooking simultaneously with the dissolving of lignin. The cooking conditions are highly acidic and the temperature varies from about 140° C. to 150° C., whereby the hydrolysis is emphasized. The result, however, is always a compromise with delignification. No high content of alpha cellulose is obtained. Another drawback is the decrease in the degree of polymerization of cellulose and yield losses, which also limit the hydrolysis possibilities. In U.S. Pat. No. 5,139,617 (Tikka and Virkola), an anthraquinone—neutral sulfite pulping process is disclosed. Various improvements have been suggested, such as modification of the cooking conditions and even a prehydrolysis step followed by an alkaline sulfite cooking stage.
The utilization of the hydrolyzed carbohydrates released in the prehydrolysis has been neglected and no commercial production based on hydrolysis material has been reported in spite of the fact that this option is mentioned in e.g. the above-referred patents. In today's industrial practice, the hydrolysate is neutralized, combined with the spent cooking liquor, evaporated and combusted in the recovery boiler of the pulp mill's energy and chemicals recovery process.
Looking closer into the reasons of neglecting any other reasonable use of the carbohydrate material uncovers practical problems: The prehydrolysis process step is most practically carried out in steam phase, introducing direct steam to the chip column in the digester. Due to the material and energy balance, very little, if any, liquid hydrolysate phase is generated, as all condensate is trapped in the porosity of the wood material. A separate washing stage using a washing liquid within the digester between the prehydrolysis and the cooking steps takes time, lowers production, is very unfavorable to the energy balance and would produce a very dilute carbohydrate solution requiring further expensive evaporation prior to any reasonable use. Another process possibility has been to carry out the prehydrolysis step in liquid phase. In this case, too, the large amount of liquid and the resulting low concentration of carbohydrates have prevented the development of any reasonable production economy. As a result, the lack of an adequate carbohydrate removal process has precluded the utilization of this renewable raw material of natural origin.
In US patent application 2005/0065336, a pulping process is disclosed, which involves mechanical treatment of wood chips and subjecting of the resulting mass to prehydrolysis using mineral acid treatment and subsequent steaming. After countercurrent washing, good yields of both alfa-cellulose and hemicellulose are reported. The process requires both further comminution of the chips and special process equipment for the hydrolysis and separation operations.